rust: str: simplify KUnit tests `format!` macro

Simplify the `format!` macro used in the tests by using
`CString::try_from_fmt` and directly `unwrap()`ing.

This will allow us to change both `unwrap()`s here in order to showcase
the `?` operator support now that the tests are KUnit ones.

Reviewed-by: David Gow <davidgow@google.com>
Acked-by: Danilo Krummrich <dakr@kernel.org>
[ Split from the next commit as suggested by Tamir. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>